Rubber composition

ABSTRACT

A rubber composition, which comprises 100 parts by weight of a rubber mixture consisting of 70-95 wt. % of solid rubber and 30-5 wt. % of liquid rubber reactive on the solid rubber, and 450-1,000 parts by weight of magnetic powder. The rubber composition has an improved processability by adding liquid rubber reactive on solid rubber to the conventional magnetic powder-containing rubber, without any deterioration of physical properties proper to the rubber, while maintaining a high magnetic force proper to the magnetic powder, keeping the lower viscosity of rubber compound and holding distinguished heat resistance and flexibility of moldings.

TECHNICAL FIELD

The present invention relates to a rubber composition, and moreparticularly to a rubber composition containing magnetic powder inaddition to rubber components.

BACKGROUND ART

In magnetic encoders for use at encoder positions of wheel speedsensors, etc., rubber magnets designed specifically for the sensors havebeen so far used. Magnetic force as the most important characteristic ofrubber magnets designed specifically for the sensors is substantiallyproportional to a mixing ratio of magnetic powder, and thus to increasethe magnetic force, the mixing ratio of the magnetic powder must beincreased, but too higher the mixing ratio of the magnetic powder givesrise to such problems as deterioration of processability due to anincrease in the viscosity of rubber compound or an increase in thehardness of moldings and loss of flexibility as deemed to be anadvantage of rubber magnet.

To solve these problems, it has been so far proposed to use aplasticizer or a processing aid, but use of a large amount ofplasticizer or processing aid, which is necessary for assuring theflexibility, also gives rise to such problems as poor heat resistancedue to extraction of the plasticizer or the processing aid anddeterioration of bondability. Thus, these methods have been found notsatisfactory from all aspects of processability, moldability andphysical properties.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a magneticpowder-containing rubber composition with improved processability, heatresistance and flexibility of moldings while maintaining a high magneticforce of the magnetic powder-containing rubber without any deteriorationof physical properties proper to the rubber.

The object of the present invention can be attained by a rubbercomposition, which comprises 100 parts by weight of a rubber mixtureconsisting of 70-95 wt. % of solid rubber and 30-5 wt. % of liquidrubber reactive on the solid rubber, and 450-1,000 parts by weight ofmagnetic powder.

As the solid rubber, nitrile rubber (NBR), acrylic rubber (ACM), ethylacrylate-ethylene copolymer rubber (AEM), ethylene-propylene copolymerrubber (EPDM), fluororubber (FKM), etc. can be used, dependent upon thedesired application. From the viewpoint of cost, NBR, ACM, AEM and EPDMare preferable, and from the viewpoint of easiness to attain a higherfilling ratio of the magnetic powder NBR is more preferable.

For the balance between the oil resistance and low temperaturecharacteristics, a solid NBR with a nitrile content of preferably35-45%, more preferably 36-42%, is used. In other words, commerciallyavailable solid NBR can be used as such. Below the lower limit ofnitrile content the oil resistance will be deteriorated, whereas abovethe upper limit of nitrile content the low temperature characteristicswill be deteriorated.

The liquid rubber reactive on the solid rubber is a liquid rubber havingthe same structure as or similar structure to that of the solid rubberand being cocross-linkable with the same vulcanizing agent as for thesolid rubber, and liquid NBR, liquid EPDM, liquid FKM, etc. can be used,dependent upon selected solid rubber. Liquid rubber with a B-typeviscosity (70° C.) of preferably 4,000-20,000 cps, more preferably4,500-15,000 cps, is used.

Liquid NBR for use in NBR as the solid rubber has preferably a B-typeviscosity (70° C.) of 4,000-8,000 cps and a nitrile content of 26-32%,more preferably a B-type viscosity (70° C.) of 4,500-7,000 cps and anitrile content of 28-30%, and commercially available liquid NBR can beused as such. Below the lower limit of B-type viscosity, the strength ofthe vulcanization products will be lowered, whereas above the upperlimit of B-type viscosity the plasticity will be lost. Below the lowerlimit of nitrile content the oil resistance will be deteriorated,whereas above the upper limit of nitrile content the low temperaturecharacteristics will be deteriorated.

The solid rubber and the liquid rubber reactive on the solid rubber areused in a proportion of 70-95 wt. % of the solid rubber to 30-5 wt. % ofthe liquid rubber reactive on the solid rubber, preferably 80-93 wt. %of the solid rubber to 20-7 wt. % of the liquid rubber reactive on thesolid rubber. Below 5 wt. % of the liquid rubber, the effect of additionof the liquid rubber reactive on the solid rubber will be lowered,whereas above 30 wt. % the viscosity of the rubber compound will be muchlowered, resulting in deterioration of the processability.

As the magnetic powder, at least one of ferrite magnets and rare earthmagnets is used usually, but preferably from the viewpoints of lowercost and higher bondability to the rubber, ferrite magnets is used,though the magnetic force is lower than those of rare earth magnets, andmore preferably from the viewpoint of the magnetic force strontiumferrite SrO.6Fe₂O₃ and barium ferrite BaO.6Fe₂O₃ are used. The magneticpowder is formed into test samples by adding 1.5 ml of an aqueous 5 wt.% PVA solution to 20 g of the magnetic powder and placing the mixture ina cylindrical mold, 25.4 mm in diameter, followed by pressing underpressure of 1 ton/cm². The resulting test samples are then subjected todetermination of green magnetism, that is remnant flux density Br andcoercive force iHc, by a direct-current magnetizing tester. It ispreferable to use a magnetic powder with a Br value of 1,600 (G) or moreand an iHc value of 3,000 (Oe) or more.

The magnetic powder is used in a proportion of 450-1,000 parts byweight, preferably 600-900 parts by weight to 100 parts by weight of therubber mixture. Below the lower limit proportion the magnetic force,when used for encoders, will not be satisfactory, whereas above theupper limit proportion the flexibility of moldings will be considerablydeteriorated.

A reinforcing agent, an antioxidant, a plasticizer, a processing aid, across-linking agent, a cross-linking aid, etc. can be added to theforegoing rubber composition, if necessary, and vulcanization can becarried out by the ordinary method so far used for rubber.

An adhesive for use in bonding of the vulcanization moldings of thepresent rubber composition to encoder metal rings includes, for example,commercially available phenol resin, epoxy resin, etc. and stainlesssteel, cold rolled steel sheets, etc. are used as the metal.

BEST MODES FOR CARRYING OUT THE INVENTION

The present invention will be described in detail below, referring toExamples, which should not be interpreted to be restrictive of thepresent invention.

EXAMPLES 1 TO 3 AND COMPARATIVE EXAMPLES 1 to 3

Example No. Com. Ex. No. 1 2 3 1 2 3 Solid NBR (N220S, product of 90 8070 100 100 100 Japan Synthetic Rubber Co. Ltd: Nitrile content: 41%)Liquid NBR (Nipol 1312, 10 20 30 0 0 0 product of Nihon ZeonCorporation: Nitrile content: 28%, B-type viscosity (70° C.): 6,000 cps)Sr ferrite (FM-201, product of 800 800 800 450 800 800 Toda Kogyo K.K.)Activated zinc oxide powder 3 3 3 3 3 3 Antioxidant (4,4′-bis(α,α- 2 2 22 2 2 dimethylbenzyl)diphenyl amine; Nocrac CD, product of Ouchi- ShinkoKagaku K.K.) Processing aid (stearic acid) 2 2 2 2 2 2 Polyether-basedplasticizer 5 5 5 5 5 20 (RS700, product of Asahi Denka Kogyo K.K.)Sulfur 0.8 0.8 0.8 0.8 0.8 0.8 Cross-linking aid 2 2 2 2 2 2(tetramethylthiuram disulfide; Nocceler TT, product of Ouchi- ShinkoKagaku K.K.) Cross-linking aid (N-cyclohexyl- 1 1 1 1 1 1 2-benzothiazylsulfenamide; Nocceler CZ, product of Ouchi- Shinko Kagaku K.K.)

The foregoing components were kneaded in a internal mixer and throughopen rolls, followed by compression molding at 170° C. for 15 minute toprepare 2 mm-thick test samples.

The test samples were subjected to tests according to the following testitems:

-   -   Normal state physical properties: according to JIS K6251 and JIS        K6253    -   Mooney viscosity Vm: according to the definition in JIS K6200        terms    -   Cracking at molding: sheets, 120 mm×240 mm×2 mm were molded and        cracking at mold releasing was observed    -   Bondability test: according to 90° peeling method described in        JIS K6256, paragraph 5    -   Magnetic force: minimum value at the center of 2 mm-thick, 29 mm        diameter samples (2 kV-1200 μF magnetization) by a handy gauss        meter    -   Heat resistance: hardness changes after heating at 120° C. for        70 hours (tests with a JIS No. 4 dumb-bell-punched test pieces)    -   Shrinkage: shrinkage in the longitudinal direction of test        pieces, 5 mm×100 mm, after heating at 175° C. for 70 hours

Results of measurements of test samples obtained in the foregoingExamples and Comparative Examples are given m the following Tabletogether with results of measurement of Mooney viscosity (125° C.) ofkneading products. TABLE Example No. Com. Ex. No. Measurement Item 1 2 31 2 3 Normal state physical properties   Hardness (Duro A) 87 81 78 86 94 80   Tensile strength (Mpa) 2.4 1.7 1.1 4.5  2.7 0.8   Elongation(%) 350 570 780 260 370 620 Mooney viscosity Vm 58 34 18 60 100> 32Cracking at molding (number of 0/10 0/10 0/10 0/10 8/10 0/10 crackedsamples/number of total samples) Bondability test   Rubber retention (%)100 100 100 100 100 60 Magnetic force (G) 108 107 107 88 105 101 Heatresistance   Hardness changes (points) +4 +7 +7 +6  +2 +10 Shrinkage (%)2.9 3.5 4.0 9.0  3.1 8.3

Industrial Utility

By adding liquid rubber reactive on solid rubber to the conventionalmagnetic powder-contained rubber, processability can be improved withoutany deterioration of physical properties proper to the rubber, whilemaintaining a high magnetic force proper to the magnetic powder, keepingthe lower viscosity of rubber compound and holding distinguished heatresistance and flexibility of moldings. The present rubber compositionscan be suitably used as rubber magnets designed specifically for sensorsin magnetic encoders used at the encoder positions such as wheel speedsensors, etc.

1-8. (canceled)
 9. A rubber magnetic for use in sensors which comprisesa rubber composition having 100 parts by weight of a rubber mixtureconsisting of 70-94 wt. % of solid rubber and 30-5 wt. % of liquidrubber reactive on the solid rubber, and 450-1,000 parts by weight ofmagnetic powder.
 10. A rubber magnet according to claim 9, which is usedas an element in a magnetic encoder.
 11. A rubber magnetic according toclaim 10, which is used as an element in a wheel spin sensor.
 12. Amagnetic encoder that comprises the rubber magnetic of claim
 9. 13. Awheel spin sensor that comprises the rubber magnetic of claim 9.